The following concept is meant to give the party something to argue about in the context of a wide-open sandbox style campaign, or at least one with more than one possible adventure at any time. Note that it includes XP awards for more than just slaughter and coin, some of which I might have just made up on the spot. Please feel free to suggest more ideas in the comments.
Double XP for single combat, including jousts, gladiatorials and other forms of combat-as-performance.
Double XP for defeating any monsters that are already the subject of Song or Tale.
Double XP for any treasure taken as spoils of war.
Half XP for books, lab paraphernalia and other items of scientific or educational nature.
Double XP for books, lab paraphernalia and other items of scientific or educational nature.
Double XP for befriending or enslaving supernatural entities.
Double XP for destroying undead.
Double XP for directly opposing the foes of the faith.
Double XP for visiting Places of Beauty holy to the faith.
Half XP for treasure not immediately donated to the church.
No XP for looting tombs of believers.
Double XP for all non-magical items featured in Song or Tale.
Double XP for all treasure recovered from use of a treasure map.
Half XP for monsters defeated, excepting backstab victims.
Double XP for visiting Places of Beauty.
Double XP for learning Songs and Tales.
Double XP for jewels and other items of aesthetic value.
Double XP for fighting drow.
Half XP for fighting fellow fairy creatures.
Double XP for gold and gems recovered.
Double XP for dwarven items recovered.
Double XP for defeating monsters inhabiting ancient dwarvish strongholds.
Double XP for visiting underground Places of Beauty.
Double XP for defeating orcs, goblins and hobgoblins.
Half XP for pearls or silver recovered.
Half XP for any voyages across water.
Half XP for visiting surface Places of Beauty.
Double XP for an items of domestic utility (a golden bowl, a bejeweled chalice, etc.)
Half XP for loot that is neither cash nor a domestic item.
Double XP for attending a Heroic Feast.
Double XP for giving gifts.
Marvel 1991: Darkhawk #1 - By complete coincidence, I'm looking at this a week after Marvel published a Darkhawk 30th anniversary special, which answers one question I had about this...